The thing that makes Mac great as well as terrible is their closed architecture. Sure, you have great security with a Mac, but because of their closed architecture, no one can write software to interface with it easily or cheaply. This bad thing has a good side: you can't write viruses for a Mac.

With Windows/IBM operating systems, since anyone can write a driver, dll, or other software device for it, you get lots of easy, cheap, versatile applications, but you also get a security nightmare. This is how Gates eared such a huge market share for Windows, by making it so that anyone could work with it easily.

The answer?

LINUX

Yea! LoL!

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By your argument Linux should be the most unsecure operating system, since anyone can look at any part of the kernel's source. If your security is based on someone not knowing how your system works, then it's lame and will be eventually cracked pretty easily.

One of the most secure algorithms known to man is RSA, which can be fully understood in about an hour if you have any kind of aptitude for math. It's uncrackable because no one knows how to efficiently factor very large integers into primes, and no increase in computing power with anything short of a quantum computer will make it crackable without some incredible breakthrough in number theory.

Anyways, the core of OSX is open source (it's Darwin). It's secure because it is built from an operating system designed from the ground up to be multi-user and secure (UNIX), which has almost 40 years of experience behind it.

Windows is a technology with maybe 17-18 years behind it, that was never designed to be what it is today.